Method for obtaining a stabilized photochromic latex, latex obtained, and application to ophthalmic optics

ABSTRACT

The invention relates to methods for obtaining the photochromic latex comprising the polymerization in aqueous emulsion of an initial polymerizable mixture, comprising one or more monomers polymerizable by free-radical mechanism and one or more photochromic compounds and the addition to the initial polymerizable mixture, during the polymerization of the initial polymerizable mixture or to the final latex of at least one agent for stabilizing the photochromic properties.  
     Such latexes can be used in the production of photochromic films. Such films may be used, for example, on optical lenses.

BACKGROUND OF THE INVENTION

[0001] The present application claims priority to French Application No.01/00343 filed Jan. 11, 2001, the entire text of which is specificallyincorporated by reference herein without disclaimer.

[0002] 1. Field of the Invention

[0003] The invention in general concerns a method for obtaining aphotochromic latex whose properties, in particular photochromic, remainstable over time, the thus stabilized photochromic latexes and their usein ophthalmic optics.

[0004] 2. Description of Related Art

[0005] The preparation of photochromic latexes by polymerization ofacrylic or methacrylic monomers comprising one or more photochromiccompounds in aqueous emulsion is already known.

[0006] These photochromic latexes are conventionally obtained bypreparing a first solution comprising the monomer(s), the photochromiccompound(s) and a second solution comprising the dispersion medium,generally water and a surface-active agent, and mixing the two solutionswith vigorous stirring to obtain a pre-emulsion. The pre-emulsion isthen transferred in a single step into a reactor and degassed undernitrogen with stirring. After the degassing, a polymerization initiator,generally in solution in a solvent (typically water), is added and thepolymerization is performed by heating to give the latex.

[0007] The latex obtained is then coarsely filtered through fabric andstored in the absence of light.

[0008] Such a method for obtaining a latex with photochromic propertiesis disclosed in the document FR 2.790.264.

[0009] The French patent application N° 00 14903 filed on Nov. 17, 2000discloses the use of mini-emulsions for preparing photochromic latexes.

[0010] More particularly, according to this method, the preparation of aphotochromic latex comprises preparing a mixture comprising at least onemonomer with a C═C group polymerizable by free-radical mechanism, atleast one organic photochromic compound, at least one surface-activeagent, water and optionally a polymerization initiator, treating themixture obtained to form a mini-emulsion comprising an organic phasedispersed in the form of droplets with a diameter of 50 to 500 nm,preferably 50 to 300 nm, in an aqueous phase, adding a polymerizationinitiator to the mini-emulsion, if this has not already been introduced,or an additional quantity of initiator compared to the quantityinitially introduced, polymerizing the reaction mixture obtained andrecovering the photochromic latex.

[0011] The initial mixture is preferably obtained by separatelypreparing a first solution comprising the monomer(s), the photochromiccompound(s) and optionally the stabilization agent(s) and a secondsolution comprising water and the surface-active agent(s), thencombining the two solutions.

[0012] The mini-emulsion is obtained for example by emulsification bymeans of a micro-fluidizer such as the micro-fluidizer disclosed in theU.S. Pat. No. 4,533,254.

[0013] The use of mini-emulsions for emulsion polymerization is alsodisclosed in the documents FR 2.785.904, EP 852.239, U.S. Pat. No.5,569,716. U.S. Pat. No. 5,653,965 and WO 98/50436.

[0014] The latexes obtained by conventional methods and the method ofpolymerization in mini-emulsion, although giving satisfactory resultswhen deposited and dried soon after their preparation, nevertheless havethe disadvantage of losing a significant amount of their photochromicproperties over time.

[0015] More exactly, analyses by high-pressure liquid-phasechromatography (HPLC) of photochromic latexes, whatever their method ofpreparation, conventional or by mini-emulsion, have shown a decrease inconcentration of the photochromic compounds within the liquid latex as afunction of time. This degradation is generally very rapid and can leadto a 50% decrease in photochromic compounds in the liquid latex in onemonth.

[0016] The problem of stabilizing photochromic compounds present in thinfilms of resins has already been tackled in the prior art, but mainly asregards stabilization against external degradations such as light, andin particular UV radiation.

[0017] Thus, the patent U.S. Pat. No. 5,914,174 discloses photochromicresins containing additives such as antioxidants, radical-trappingagents and UV absorbers. In particular, it is stated in this patent thatthe use of hindered amines and agents which remove species in theexcited state improves performances against bad weather.

[0018] In the case of latexes, the technical problem is that of thedegradation of photochromic compounds within liquid latexes, in otherwords before their application as a film, and even in the absence of anyexternal factor likely to modify or degrade the properties of thephotochromic compounds.

[0019] It is particularly significant to note that the degradation ofphotochromic latexes occurs even when they are stored in closedcontainers, in the absence of ambient air or light. This is thus aproblem completely specific to latexes.

SUMMARY OF THE INVENTION

[0020] The object of the invention is thus a method for obtaining aphotochromic latex whose properties, in particular photochromic, remainstable over time, in particular when the latex is stored in the absenceof ambient air and of light.

[0021] A further object of the invention is a stabilized photochromiclatex, whose photochromic properties remain stable over time, inparticular when the latex is stored in the absence of ambient air and oflight.

[0022] An additional object of the invention is substrates, inparticular optical articles such as ophthalmic lenses, coated with driedfilms of the latexes such as those defined above.

[0023] The above objectives are achieved according to the invention by amethod for obtaining photochromic latexes comprising a polymerization inaqueous emulsion of an initial polymerizable mixture comprising one ormore organic monomers containing C═C groups which are polymerizable byfree-radical mechanism and one or more photochromic compounds until afinal latex is obtained, characterized in that it comprises the additionto the initial polymerizable mixture, during the polymerization of theinitial polymerizable mixture or to the final latex of an effectivequantity of at least one stabilization agent selected from compounds ofcyclopentene, cyclohexene, cycloheptene, cyclooctene and compoundscontaining an ethylenic unsaturation not forming part of an aromaticring and which contain, in the α position with respect to the ethylenicunsaturation, a carbon atom bearing a free hydroxy group.

[0024] The stabilization agent is preferably introduced into the initialpolymerizable mixture.

[0025] In the present application, final latex should be understood tomean a latex whose concentration in the dry extract does not change overtime. The final latex is generally obtained after about 1 hour ofreaction.

[0026] The invention also concerns a stabilized photochromic latexcomprising a dispersion in an aqueous phase of polymer particlesresulting from the polymerization in aqueous phase of a mixture of oneor more organic monomers containing C═C groups which are polymerizableby free-radical mechanism and one or more photochromic compounds and aneffective quantity of at least one stabilization agent selected fromcompounds of cyclopentene, cyclohexene, cycloheptene, cyclooctene andcompounds containing an ethylenic unsaturation not forming part of anaromatic ring and which contain, in the cc position with respect to theethylenic unsaturation, a carbon atom bearing a free hydroxy group.

[0027] The agents for stabilizing the photochromic properties accordingto the invention are compounds known as anti-yellowing agents informulations based on bisphenol A dimethacrylates or bisphenol A diallylcarbonate. These agents are disclosed, amongst others, in U.S. Pat. Nos.5,702,825 and EP 224.123.

[0028] More particularly, these stabilization agents are compounds ofcyclopentene, cyclohexene, cycloheptene, cyclooctene and compoundscontaining an ethylenic unsaturation not forming part of an aromaticring and which contain, in the α position with respect to the ethylenicunsaturation, a carbon atom bearing a free hydroxy group.

[0029] The preferred stabilization agents are the derivatives ofcyclohexene.

[0030] A wide variety of cyclohexene compounds may be used asstabilization agents. The cyclohexene compounds may be represented bythe formula:

[0031] in which each Y is independently an alkyl group of 1 to 4 carbonatoms, Z is a hydroxy group, a 2-oxoethyl group, a hydroxyalkyl group of1 to 3 carbon atoms, an alkoxycarbonyl group of 2 to 5 carbon atoms, or

[0032] in which R′ is an alkyl radical of 1 to 4 carbon atoms and R″ isan alkane di-yl radical of 2 to 4 carbon atoms or an alkylidene radicalof 1 to 5 carbon atoms, y is an integer from 0 to 3, z represents 0 or1, w represents 0 or 1, and the sum of z and w is 0 or 1.

[0033] Y is preferably a methyl group. The groups represented by Y maybe identical or different.

[0034] When Z is a hydroxyalkyl group, it is typically a hydroxymethyl,2-hydroxyethyl, or 1-hydroxy-1-methylethyl group. When Z is analkoxycarbonyl group, it typically contains 2 or 3 carbon atoms. Thepreferred group is a methoxycarbonyl group. R′ is preferably a methyl,ethyl or propyl group. When R″ is an alkane di-yl group, it may belinear or branched. The preferred group is a ethane di-yl group. When R″is an alkylidene group, it is conventionally a methylene group or amethyl ethylidene group. y is preferably equal to 0 or 1. z is similarlypreferably equal to 0 or 1. Also preferably, w is equal to 0.

[0035] Examples of cyclohexene compounds which may be used in theinvention include cyclohexene, α-terpineol, terpinen-4-ol, α-terpinylacetate, α-terpinyl propionate, α-terpinyl butyrate,1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, 4-methyl-1-cyclohexene,methyl 1-cyclohexene-1-carboxylate, 3-methyl-2-cyclohexen-1-ol,3-methyl-2-cyclohexen-1-one, 4-isopropyl-2-cyclohexen-1-one,3,5-dimethyl-2-cyclohexen-1-one, 4,4-dimethyl-2-cyclohexen-1-one,isophorone, 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, and3,5,5-trimethyl-2-cyclohexen-1-ol. The preferred cyclohexene derivativesare cyclohexene, α-terpinyl acetate, α-terpinyl propionate, α-terpinylbutyrate, and most particularly cyclohexene, α-terpinyl acetate or amixture of these.

[0036] The compounds with ethylenic unsaturation which are suitable forthe invention contain, as stated above, an ethylenic unsaturation notforming part of an aromatic ring and containing in the α position withrespect to the ethylenic unsaturation a carbon atom bearing a hydroxygroup and bonded to at least one hydrogen atom.

[0037] These compounds with ethylenic unsaturation are preferablyselected from the compounds of formula:

[0038] in which

[0039] R¹, R² represent hydrogen or one of the R¹ and R² groupsrepresents hydrogen, while the other forms, with the R⁵ group, a C₅-C₁₀ring which may be substituted with one or more linear or branched C₁-C₄alkyl groups or one or more C₂-C₄ alkylene groups; or one of the R¹, R²groups may represent hydrogen, while the other represents a C₁-C₆aliphatic alkyl group, a C₄-C₁₆ cyclic hydrocarbon group or a C₆-C₁₆aromatic hydrocarbon group.

[0040] R³, R⁴ and R⁵ independently represent hydrogen or a C₁-C₆ alkylgroup which may be substituted with one or more OH groups and in whichthe chain may be interrupted by one or more ether, ester or ketonegroups; a C₆-C₁₆ aromatic hydrocarbon group or a C₄-C₁₆ cyclichydrocarbon group; R⁵ may form, with one of the R¹ or R² groups, aC₅-C₁₀ ring which may be substituted with one or more linear or branchedC₁-C₄ alkyl groups or one or more C₂-C₄ alkylene groups;

[0041] R³ and R⁴ may form a C₅-C₁₀ ring which may be substituted withone or more C₁-C₄ alkyl groups or C₂-C₄ alkylene groups;

[0042] Two or more carbon atoms of the C₅-C₁₀ ring formed with R¹ or R²and R⁵ or with R³ and R⁴ may be bonded by a hydrocarbon bridge.

[0043] According to the invention, the C₁-C₆ alkyl group is preferably amethyl or ethyl group.

[0044] Preferably R¹ and R² represent hydrogen, R³ represents a methylgroup, R⁴ represents hydrogen or a methyl group and R⁵ representshydrogen, a methyl, ethyl or CH₂OH group.

[0045] The preferred compounds with ethylenic unsaturation of formula(II) are allyl alcohol, methallyl alcohol, crotyl alcohol,2-cyclohexen-1-ol, trans-2-hexen-1-ol, cis-2-butene-1,4-diol,3-methyl-2-buten-1-ol, trans-2-methyl-3-phenyl-2-propan-1-ol,3-buten-2-ol, carveol, myrtenol, verbenol and trans-cinnamyl alcohol.

[0046] The preferred compound with ethylenic unsaturation is3-methyl-2-buten-1-ol (MBOL).

[0047] As stated above, the stabilization agent or agents forphotochromic properties may be introduced at any time during the latexpreparation, and in particular into the initial monomer mixture or thefinal latex, in other words at the end of the latex synthesis, when thedry extract obtained is stable and no longer changes over the course oftime. This end of synthesis generally occurs after 1 hour of reaction.

[0048] However, the introduction of the stabilization agent or agentsfor photochromic properties is preferably performed into the initialpolymerizable mixture.

[0049] The quantity of stabilization agent introduced may vary over awide range, but must be at least sufficient to obtain a significantstabilization of the photochromic properties of the final latex.

[0050] The quantity of stabilization agent or agents introducedgenerally represents 0.1 to 10% by weight, preferably 1 to 10% by weightand more preferably about 5% by weight compared to the weight ofmonomers in the initial mixture.

[0051] The monomers polymerizable by free-radical mechanism suitable forthe present invention may be aromatic or non-aromatic.

[0052] The recommended monomers are monomers of the alkyl (meth)acrylatetype, preferably of the mono(meth)acrylate type.

[0053] The alkyl groups are preferably C₁-C₁₀ alkyl groups, such asmethyl, ethyl, propyl and butyl.

[0054] The preferred monomers include the methyl, ethyl, propyl, butyl,and isobutyl acrylates and methacrylates and also the bifunctionalderivatives such as butanediol di(meth)acrylate or trifunctional such astrimethylol propane tri(meth)acrylate.

[0055] The preferred aromatic monomers include monofunctional aromaticcompounds such as phenyl (meth)acrylate, benzyl (meth)acrylate,phenoxyethyl (meth)acrylate and styrene.

[0056] Mixtures of these monomers may also be used, in particularmixtures of C₂-C₁₀ alkyl monomers and C₁-C₃ alkyl methacrylates.

[0057] The organic photochromic compounds suitable for the presentinvention are all organic compounds showing photochromic properties.These compounds are well known in the state of the art.

[0058] The preferred compounds are the chromenes and spirooxazines.

[0059] The chromenes are described, amongst others, in the documentsU.S. Pat. No. 3,567,605, U.S. Pat. No. 5,066,818, U.S. Pat. No.5,645,767, U.S. Pat. No. 5,656,206, WO 93/17071, WO 94/20869, FR2.688.782, FR 2.718.447, EP 0.401.958, and EP 0.562.915.

[0060] The spirooxazines are also well known photochromic compounds.Such compounds are described, amongst others, in the patents U.S. Pat.No. 5,114,621, EP 0.245.020, JP A 03251587, WO 96/04590 and FR2.763.070.

[0061] The photochromic compound is introduced in sufficient quantity toobtain the desired photochromic effect in the final latex films.

[0062] The concentrations in photochromic compounds generally vary from1 to 10%, preferably from 2 to 7% by weight compared to the weight ofpolymerizable monomers present in the latex.

[0063] To improve the solubilization of the photochromic compound, asmall quantity of a solvent of the photochromic compound may optionallybe added to the mixture, for example N-methylpyrrolidone in the case ofthe chromenes. The quantity of solvent added may be up to 20% by weight(depending on the solubility of the compound) compared to the weight ofpolymerizable monomers present in the initial mixture.

[0064] As is also conventional, the initial polymerizable mixture maycontain a surface-active agent. The surface-active agent may be ionic,non-ionic or amphoteric. The ionic surface-active agents include sodiumdodecyl sulfate, sodium dodecyl benzenesulfonate, sodium sulfonate, thesulfates of ethoxylated fatty alcohols and cetyltrimethylammoniumbromide (CTAB), azobiscyanopentanoic acid (dissolved and neutralized).

[0065] The non-ionic surface-active agents include the ethoxylated fattyalcohols.

[0066] A mixture of surface-active agents may obviously be used in theinitial polymerization mixture.

[0067] The initial polymerization mixture also contains a polymerizationinitiator.

[0068] The polymerization initiator may be any conventionally usedinitiator. It may be soluble in water or the organic phase.

[0069] The water-soluble initiators used for the polymerization of thecompositions according to the invention are salts and compounds havingat least one hydrophilic function.

[0070] These salts and compounds include the alkali metal and ammoniumpersulfates, in particular sodium and potassium persulfate, hydrogenperoxide, 2,2′-azobis(2-amidinopropane) dihydrochloride.

[0071] Partially water-soluble peroxides such as succinic peracid andt-butyl hydroperoxide may also be used.

[0072] Redox systems such as the persulfates combined with a ferrous ionmay also be used.

[0073] Cumyl hydroperoxide or hydrogen peroxide, in the presence offerrous, sulfite or bisulfite ions may also be used.

[0074] The initiators soluble in the organic phase includeazobisisobutyronitrile (AIBN). The initiator may be added to the initialpolymerizable mixture in a single step, but it is also possible to addit semi-continuously during the polymerization reaction.

[0075] The aqueous phase of the latex may contain water only or it maycontain a mixture of water and an appropriate solvent, for example toaid film formation. The aqueous phase may also contain a buffer, forexample NaHCO₃. When a solvent is present, it may generally represent upto 10% by weight of the aqueous phase, preferably less than 5% by weightand more preferably mess than 2% by weight.

[0076] The aqueous phase preferably contains water only.

[0077] As stated above, the emulsion polymerization of the initialpolymerizable mixture of monomers may be performed by conventional meansor by the technique called “mini-emulsion”.

[0078] When the mini-emulsion technique is used, an agent forstabilizing the mini-emulsion may also be added to the initialpolymerizable mixture.

[0079] This stabilization agent of a mini-emulsion may be an alkane, ahalogenated alkane or a monomer, polymerizable or not, containing afatty chain such as a fatty alcohol or a fatty alcohol ester.

[0080] The preferred stabilization agents are hexadecane, cetyl alcoholand stearyl methacrylate.

[0081] The particularly preferred stabilization agent is stearylmethacrylate since it is integrated into the network, because of thepresence of the methacrylate function.

[0082] The concentration of stabilization agents in the mixturegenerally varies from 0.1 to 10%, preferably from 2 to 6% by weightcompared to the weight of polymerizable monomers present in the initialmixture.

[0083] The deposition of the latexes according to the invention on thesubstrates may be performed by any conventional method centrifugation,spraying or by combining both methods.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0084] In the following examples, except where otherwise stated, allpercentages and parts are by weight.

EXAMPLES 1 TO 5 AND COMPARATIVE EXAMPLE A

[0085] Agents of the methyl-2-buten-1-one and cyclohexene type wereintroduced into the formulations (5% by weight compared to the totalweight of monomer in the initial mixture).

[0086] The compositions of the initial polymerizable mixtures used aregiven in table I below: TABLE I Composition of the initial polymerizablemixture N° M1 M2 M3 M4 M5 M6 M7 Polymerizable monomer 46.4 46.4 46.446.4 46.4 46.4 46.4 Butyl methacrylate (g) Photochromic compound SpiroA* (g) 3.25 3.25 3.25 3.25 3.25 — — PC1** (g) — — — — — 3.25 — SpiroB*** (g) — — — — — — 3.25 Photochromic property — 2.32 — 2.32 — — —stabilization agent MBOL(g) — — — — 2.32 2.32 — Cyclohexene (g) — — — —— — 2.32 Terpinyl acetate Mini-emulsion stabilization 2.32 — — 2.32 2.322.32 2.32 agent**** Stearyl methacrylate (g) Suface-active agent 1.5 1.51.5 1.5 1.5 1.5 1.5 CTAB (g) Initiator***** 0.48 0.48 0.48 0.48 0.480.48 0.48 2,2′-azobis(2-amidinopropane) 2HCl (g) Water 73 73 73 73 73 7373 *Spiro A

**PC1 Mixture of 2-naphthopyrans according to U.S. Pat. Nos. 5,645,767and 5,656,206. ***Spiro B

****Not used in the conventional method. Only used for themini-emulsion. *****Solution of 0.48 g, diluted in 4.8 g of water.

[0087] The latexes were prepared either by a conventional emulsionpolymerization method, or by a mini-emulsion polymerization method.

[0088] The initial mixtures were prepared by combining a solution Acontaining the polymerizable monomer, optionally the photochromicproperty stabilization agent and the mini-emulsion stabilization agentand the photochromic compound and a solution B containing thesurface-active agent and water.

[0089] The conventional emulsions were obtained by simple mechanicalstirring of the mixtures.

[0090] To obtain the mini-emulsions, the mixtures were treated in asingle batch in a micro-fluidizer from STANSTED FLUID POWER LTD, modelnm-CEN 7400H at a pressure of 70 MPa.

[0091] The conventional emulsions and mini-emulsions were thentransferred into a reactor and subjected to degassing under nitrogen for30 minutes at 60° C., with stirring at 250 r.p.m.

[0092] A solution of an initiator was then prepared by dissolving 0.48 gof 2,2′-azobis(2-amidinopropane) dihydrochloride in 4.8 g of water, andthis solution was added in a single step into the reactor containing 103g of emulsion or mini-emulsion to initiate the polymerization reaction.

[0093] The polymerization was performed at a temperature of 60° C.

[0094] At the end of the polymerization, the heating was stopped and themixture left to cool to ambient temperature (23° C.), and optionally theappropriate quantity of photochromic property stabilization agent wasadded when this had not been added in the initial polymerizable mixture.

[0095] The conditions of preparation of the latexes of the comparativeexamples A and 1 to 5 are given in Table II below: TABLE II Addition ofphotochromic property stabilization agent to the final latex InitialCyclohexene Example N° mixture Emulsion type MBOL (g) (g) Comparative AM1 Mini-emulsion — — 1 M2 Conventional — — 2 M3 Conventional 2.32 — 3 M4Mini-emulsion — — 4 M5 Mini-emulsion — — 5 M6 Mini-emulsion — — 6 M7Mini-emulsion — —

[0096] The variation of the latexes obtained was monitored by HPLCdetermination of the photochromic colorant.

[0097] The procedure was that several samples of the latexes were takenover a period of time, and these samples were deposited by spin coating(200 r.p.m. for 10 seconds, then 1000 r.p.m. for 10 seconds) onto SIGMALlenses and dried in an oven at 50° C. to give transparent photochromicfilms of thickness 3.7 μm.

[0098] The films were recovered and the photochromic compounds wereextracted with 10 ml of acetonitrile at 80° C. in the presence of aKEMIX n° 16 internal standard. This extraction was carried out for 20mm. After stirring, the solution was filtered, then analysed byinverse-phase high-pressure liquid-phase chromatography (HPLC). Thechromatography was performed on a KROMASIL® C18 column (TOUZART andMATIGNON).

[0099] The elution solvent was an acetonitrile/water mixture (80/20) inisocratic elution mode or in gradient mode and the elution speed was 1ml/minute. The detector used was a WATERS 484 detector at 230 nm.

[0100] The yield (%) of photochromic compounds was the ratio of thequantity of photochromic compound found in the film compared to thequantity theoretically expected.

[0101] The results of these tests are summarized in Table III below:TABLE III Concentration of Example N° Date of analysis photochromiccompound (%) Comparative A Mar. 2, 2000 6 Mar. 29, 2000 4.08 May 11,2000 2.5 In form of dried film Mar. 29, 2000 6 prepared Feb. 29, 2000May 11, 2000 6 1 Mar. 29, 2000 6 May 11, 2000 5.6 2 Mar. 29, 2000 6 May11, 2000 3.3 3 Jun. 23, 2000 5.3 Aug. 2, 2000 4.8 Sep. 9, 2000 3.3 4Jun. 23, 2000 5.4 Aug. 2, 2000 5.4 Sep. 12, 2000 5.4 5 Aug. 18, 2000 4.1Sep. 12, 2000 4 6 Nov. 2, 2000 5.9 Dec. 8, 2000 5.6

[0102] The results in Table III show that the concentration ofphotochromic compound in the latex of comparative example A, which didnot contain a photochromic property stabilization agent according to theinvention, reduced considerably, from 6 to 2.5% in a little over twomonths.

[0103] It is interesting to note that when the same latex was applied inthe form of a film and dried, a substrate coated with the film remainedstable over the same period of time (concentration of photochromiccompound stable at 6%). This confirms that the degradation of thephotochromic compound does indeed occur in the liquid latex.

[0104] Examples 1 to 5 clearly show that the addition of an agentaccording to the invention stabilized the concentration of photochromiccompound in the liquid latex over periods of at least two months.

[0105] The results also show that the addition of the photochromicproperty stabilization agent is preferably performed at the beginning ofthe polymerization, given that this does not alter the polymerizationkinetics.

[0106] In addition, the cyclohexene agents seemed to lead to longerstabilization periods, since a significant degradation of thephotochromic properties appeared after two months with MBOL, which wasnot the case with cyclohexene.

[0107] Finally, cyclohexene has the advantage of not giving any residualcolour to the film.

1. Method for obtaining a photochromic latex comprising a polymerizationin aqueous emulsion of an initial polymerizable mixture comprising oneor more organic monomers containing C═C groups which are polymerizableby free-radical mechanism and one or more photochromic compounds until afinal latex is obtained, characterized in that it comprises the additionto the initial polymerizable mixture, during the polymerization of theinitial polymerizable mixture or to the final latex of an effectivequantity of at least one agent for stabilizing the photochromicproperties selected from compounds of cyclopentene, cyclohexene,cycloheptene, cyclooctene and compounds containing an ethylenicunsaturation not forming part of an aromatic ring and which contain, inthe α position with respect to the ethylenic unsaturation, a carbon atombearing a free hydroxy group.
 2. Method according to claim 1,characterized in that the stabilization agent is a compound ofcyclohexene.
 3. Method according to claim 2, characterized in that thecyclohexene compounds correspond to the formula:

in which each Y is independently an alkyl group of 1 to 4 carbon atoms,Z is a hydroxy group, a 2-oxoethyl group, a hydroxyalkyl group of 1 to 3carbon atoms, an alkoxycarbonyl group of 2 to 5 carbon atoms, or aR′COOR″ group in which R′ is an alkyl radical of 1 to 4 carbon atoms andR″ is an alkane di-yl radical of 2 to 4 carbon atoms or an alkylideneradical of 1 to 5 carbon atoms, y is an integer from 0 to 3, zrepresents 0 or 1, w represents 0 or 1, and the sum of z and wis 0 or 1.4. Method according to claim 3, characterized in that the cyclohexenecompounds are selected from cyclohexene, α-terpineol, terpinen-4-ol,α-terpinyl acetate, α-terpinyl propionate, α-terpinyl butyrate,1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, 4-methyl-1-cyclohexene,methyl 1-cyclohexene-1-carboxylate, 3-methyl-2-cyclohexen-1-ol,3-methyl-2-cyclohexen-1-one, 4-isopropyl-2-cyclohexen-1-one,3,5-dimethyl-2-cyclohexen-1-one, 4,4-dimethyl-2-cyclohexen-1-one,isophorone, 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, and3,5,5-trimethyl-2-cyclohexen-1-ol.
 5. Method according to claim 3,characterized in that the cyclohexene compound is cyclohexene.
 6. Methodaccording to claim 1, characterized in that the carbon in the α positionof the compound with ethylenic unsaturation is bonded to at least onehydrogen atom.
 7. Method according to claim 6, characterized in that thecompounds with ethylenic unsaturation correspond to the formula:

in which R¹ and R² represent hydrogen or one of the R¹ and R² groupsrepresents hydrogen, while the other forms, with the R⁵ group, a C₅-C₁₀ring which may be substituted with one or more linear or branched C₁-C₄alkyl groups or one or more C₂-C₄ alkylene groups; or one of the R¹, R²groups may represent hydrogen, while the other represents a C₁-C₆aliphatic alkyl group, a C₄-C₁₆ cyclic hydrocarbon group or a C₆-C₁₆aromatic hydrocarbon group; R³, R⁴ and R⁵ independently representhydrogen or a C₁-C₆ alkyl group which may be substituted with one ormore OH groups and in which the chain may be interrupted by one or moreether, ester or ketone groups; a C₆-C₁₆ aromatic hydrocarbon group or aC₄-C₁₆ cyclic hydrocarbon group; R⁵ may form, with one of the R¹ or R²groups, a C₅-C₁₀ ring which may be substituted with one or more linearor branched C₁-C₄ alkyl groups or one or more C₂-C₄ alkylene groups; R³and R⁴ may form a C₅-C₁₀ ring which may be substituted with one or moreC₁-C₄ alkyl groups or C₂-C₄ alkylene groups; Two or more carbon atoms ofthe C₅-C₁₀ ring formed with R¹ or R² and R⁵ or with R³ and R⁴ may bebonded by a hydrocarbon bridge.
 8. Method according to claim 7,characterized in that R¹ and R² represent hydrogen, R³ represents amethyl group, R⁴ represents hydrogen or a methyl group and R⁵ representshydrogen, a methyl, ethyl or CH₂OH group.
 9. Method according to claim7, characterized in that the compound of formula (II) is selected fromthe group composed of allyl alcohol, methallyl alcohol, crotyl alcohol,2-cyclohexen-1-ol, trans-2-hexen-1-ol, cis-2-butene-1,4-diol,3-methyl-2-buten-1-ol, trans-2-methyl-3-phenyl-2-propen-1-ol,3-buten-2-ol, carveol, myrtenol, verbenol and trans-cinnamyl alcohol.10. Method according to claim 9, characterized in that the compound offormula (II) is 3-methyl-2-buten-1-ol.
 11. Method according to any ofthe preceding claims, characterized in that the quantity of photochromicproperty stabilization agent added represents 0.1 to 10% by weight,preferably 1 to 10% by weight and more preferably about 5% by weightcompared to the weight of monomers in the initial mixture.
 12. Methodaccording to any of the preceding claims, characterized in that theaqueous emulsion of the initial polymerizable mixture is amini-emulsion.
 13. Method according to any of the preceding claims,characterized in that the monomers polymerizable by free-radicalmechanism are selected from the alkyl acrylates, the alkyl methacrylatesand their mixtures.
 14. Method according to any of the preceding claims,characterized in that the photochromic compound is selected from thechromenes, the spirooxazines and their mixtures.
 15. Method according toany of the preceding claims, characterized in that the photochemicalproperty stabilization agent is added to the initial polymerizablemixture.
 16. Stabilized photochromic latex comprising an aqueousdispersion of polymer particles resulting from the free-radicalpolymerization of one or more organic monomers containing C═C groups andone or more photochromic compounds, characterized in that it contains aneffective quantity of at least one agent for stabilizing thephotochromic properties selected from compounds of cyclopentene,cyclohexene, cycloheptene, cyclooctene and compounds containing anethylenic unsaturation not forming part of an aromatic ring and whichcontain, in the a position with respect to the ethylenic unsaturation, acarbon atom bearing a free hydroxy group.
 17. Latex according to claim16, characterized in that the stabilization agent is a compound ofcyclohexene.
 18. Latex according to claim 17, characterized in that thecyclohexene compounds correspond to the formula:

in which each Y is independently an alkyl group of 1 to 4 carbon atoms,Z is a hydroxy group, a 2-oxoethyl group, a hydroxyalkyl group of 1 to 3carbon atoms, an alkoxycarbonyl group of 2 to 5 carbon atoms, or aR′COOR″ group in which R′ is an alkyl radical of 1 to 4 carbon atoms andR″ is an alkane di-yl radical of 2 to 4 carbon atoms or an alkylideneradical of 1 to 5 carbon atoms, y is an integer from 0 to 3, zrepresents 0 or 1, w represents 0 or 1, and the sum of z and w is 0or
 1. 19. Latex according to claim 18, characterized in that thecyclohexene compounds are selected from cyclohexene, α-terpineol,terpinen-4-ol, α-terpinyl acetate, α-terpinyl propionate, α-terpinylbutyrate, 1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene,4-methyl-1-cyclohexene, methyl 1-cyclohexene-1-carboxylate,3-methyl-2-cyclohexen-1-ol, 3-methyl-2-cyclohexen-1-one,4-isopropyl-2-cyclohexen-1-one, 3,5-dimethyl-2-cyclohexen-1-one,4,4-dimethyl -2-cyclohexen-1-one, isophorone,2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, and3,5,5-trimethyl-2-cyclohexen-1-ol.
 20. Latex according to claim 18,characterized in that the cyclohexene compound is cyclohexene.
 21. Latexaccording to claim 17, characterized in that the carbon in the αposition of the compound with ethylenic unsaturation is bonded to atleast one hydrogen atom.
 22. Latex according to claim 17, characterizedin that the compounds with ethylenic unsaturation correspond to theformula:

in which R¹ and R² represent hydrogen or one of the R¹ and R² groupsrepresents hydrogen, while the other forms, with the R⁵ group, a C₅-C₁₀ring which may be substituted with one or more linear or branched C₁-C₄alkyl groups or one or more C₂-C₄ alkylene groups; or one of the R¹, R²groups may represent hydrogen, while the other represents a C₁-C6aliphatic alkyl group, a C₄-C₁₆ cyclic hydrocarbon group or a C₆-C₁₆aromatic hydrocarbon group; R³, R⁴ and R⁵ independently representhydrogen or a C₁-C₆ alkyl group which may be substituted with one ormore OH groups and in which the chain may be interrupted by one or moreether, ester or ketone groups; a C₆-C₁₆ aromatic hydrocarbon group or aC₄-C₁₆ cyclic hydrocarbon group; R⁵ may form, with one of the R¹ or R²groups, a C₅-C₁₀ ring which may be substituted with one or more linearor branched C₁-C₄ alkyl groups or one or more C₂-C₄ alkylene groups; R³and R⁴ may form a C₅-C₁₀ ring which may be substituted with one or moreC₁-C₄ alkyl groups or C₂-C₄ alkylene groups; Two or more carbon atoms ofthe C₅-C₁₀ ring formed with R¹ or R² and R⁵ or with R³ and R⁴ may bebonded by a hydrocarbon bridge.
 23. Latex according to claim 22,characterized in that R¹ and R² represent hydrogen, R³ represents amethyl group, R⁴ represents hydrogen or a methyl group and R⁵ representshydrogen, a methyl, ethyl or CH₂OH group.
 24. Latex according to claim22, characterized in that the compound of formula (II) is selected fromthe group composed of allyl alcohol, methallyl alcohol, crotyl alcohol,2-cyclohexen-1-ol, trans-2-hexen-1-ol, cis-2-butene-1,4-diol,3-methyl-2-buten-1-ol, trans-2-methyl-3-phenyl-2-propen-1-ol,3-buten-2-ol, carveol, myrtenol, verbenol and trans-cinnamyl alcohol.25. Latex according to claim 24, characterized in that the compound offormula (II) is 3-methyl-2-buten-1-ol.
 26. Latex according to any ofclaims 16 to 25, characterized in that the quantity of photochromicproperty stabilization agent present in the latex represents 0.1 to 10%by weight, preferably 1 to 10% by weight and more preferably about 5% byweight compared to the weight of monomers in the initial mixture. 27.Latex according to any of claims 16 to 26, characterized in that thepolymer is an alkyl polyacrylate, an alkyl polymethacrylate or acopolymer of alkyl acrylate and alkyl methacrylate.
 28. Latex accordingto any of claims 16 to 27, characterized in that the photochromiccompound is selected from the chromenes, the spirooxazines and theirmixtures.
 29. Substrate coated with a film formed by drying a latexaccording to any of claims 16 to
 28. 30. Substrate according to claim29, characterized in that consists of an ophthalmic lens.